Hello simon1,
If you are starting your motor in a cold climate it will need to spin over fairly smartly to acheive easy starting and the additional bit if belt friction may be preventing this. Check that the pulley groove profiles are not visibly worn, as new belts in worn grooves could cause extra friction load. Excess belt tension is another cause of power loss. Run just tight enough to drive a full load without slip. One solution could be to set the belts slack for starting and then tension them with an idler pulley on the non-drive side.
Does your engine have an oil priming cup on the head by the intake port? For easier cold starting fill the cup with engine oil and push it into the intake to raise compression for cold starts. this may be enough to make a difference.
If your setup is not factory original, check that (a) pulley ratios are correct to give correct peripheral speed for your blade size,
(b) pulley sizes are at least minimum recommended for belt section used.
A section pulleys minimum 4", but 6" is better.
B section pulleys minimum 6", bigger is better.
Hope this helps, Regards, Combustor.